INTRODUCTION. Xvii 



stem as it was in its infancy. The leaf, with its 

 stalk or petiole, is composed of transparent, colour- 

 less cuticle, enclosing bundles of woody fibre and 

 spiral vessels, which expand and form a network, 

 the interstices of which are filled up with pulpy 

 cellular tissue, usually of a green colour, and 

 called parenchyma. The lower end of the petiole 

 is articulated to the bark, and the leaf itself differs 

 from the solid part of the trunk by being merely 

 of temporary duration, usually falling off* with a 

 clean fracture before it begins to decay. If, 

 however, a branch be severed from the trunk, 

 or vitality be suddenly destroyed in any other 

 way, the leaves generally lose the power of throw- 

 ing themselves off*. Hence we may frequently 

 see, in winter, a broken branch hanging from a 

 tree still retaining its withered foliage, when not 

 a leaf is to be seen on the healthy branches. 

 Leaves have been called the lungs of a plant, not 

 from any resemblance in shape which they may 

 be supposed to bear to that organ, but from a 

 similarity in their functions. In the lungs of 

 animals the blood is exposed to the action of at- 

 mospheric air, the oxygen of which is retained, 

 and carbonic acid gas is respired, the blood it- 

 self being thus converted into proper nourish- 

 ment for the animal frame : and the leaves of 

 plants absorb through pores in their cuticle 

 carbonic acid gas, which they in like manner 

 decompose ; carbon, the principal component of 

 woody fibre, is deposited, and pure oxygen re- 

 stored to the air. By this arrangement of the 

 all-wise Creator of the universe, animals are in- 

 cessantly breathing out a gas deleterious to them- 

 selves but essentia] to the growth of vegetables ; 



h 



